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Old Mar 2, 2010, 6:31 PM   #1
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Default Circular or linear Polarizing filters?

What exactly is a circular polarizing filter? I see reference to needing the circular polarizing filter with auto-focus lenses. Can a linear filter be used if I focus manually?
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Old Mar 2, 2010, 7:32 PM   #2
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A Polarizing Filter reduces reflections and glare. A linear polarizing filter interferes with autoexposure and especially autofocus systems in cameras. A circular polarizing filter doesn't.

See http://en.wikipedia.org/wiki/Polarizing_filter
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Old Mar 2, 2010, 7:34 PM   #3
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Iowa Jim,

This is from the Luminous Landscape website:

http://www.luminous-landscape.com/tu...larizers.shtml


Circular Vs. Linear Polarizers

There are two types of polarizing filters available linear or circular. Linear polarizers are more effective and less expensive than circular ones. But circular polarizers are needed with just about any camera that has a through-the-lens metering system, or autofocus.
The reason for this is that both of these systems use semi-silvered mirrors to siphon off some of the light coming though the lens. If that light is linearly polarized it renders either the metering or the autofocus ineffective. This means that you're going to have to buy circular polarizers unless you're shooting with a pre-1970's camera, or a view camera.

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Old Mar 2, 2010, 7:35 PM   #4
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Quote:
Originally Posted by TCav View Post
A Polarizing Filter reduces reflections and glare. A linear polarizing filter interferes with autoexposure and especially autofocus systems in cameras. A circular polarizing filter doesn't.

See http://en.wikipedia.org/wiki/Polarizing_filter
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Old Mar 2, 2010, 7:56 PM   #5
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That makes sense now. Thanks!
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Old Mar 2, 2010, 10:26 PM   #6
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Some practical experience - I have used a linear polarizer (bought for a film camera) with a dSLR using a manual focus lens and found the metering off sometimes (it wasn't consistent) and the focus harder to spot (and my camera didn't always tell me that the scene was in focus, which it usually does, even with manual focus lenses). After my experience, I wouldn't buy one for use with a dSLR - if you already have one you can play with it some and see the limitations.
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Old Mar 3, 2010, 1:39 PM   #7
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I don't have one already, but was about to make the purchase when I saw a reference to making sure it was compatible with my camera. I couldn't quite put my mind around the description of a circular polarizer, but it makes sense to polarize and then scatter the light all over again. Clever stuff!
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